So, one of my PCs is a Lawful Neutral Knight, coming from a small landless noble family. From the beginning of the campaign he told me that he wanted a lord to serve under, and to enhance the prestige of his family. However, later on, he directly told me OOC that he was more or less a greedy bastard who cared only for power. That's fine as a character, but he swore oaths to numerous NPCs that he never intended to keep, declared his loyalty to the kingdom and its people, and in general deceived more or less everyone he met to his true intentions, all without rolling Bluff checks (I don't think he has a bluff bonus, if he does it's something like +3, so a passive bluff, if that's a thing, might not dupe an ogre). Is this a legitimate thing for him to do?
I'm going to take a slight detour and address two sub-parts of your question before providing an answer, in order to provide some perspective the other answers may be lacking.
Lying isn't the same as bluffing.
The key difference between lying and bluffing is that when you lie, you make a statement you know is untrue, but when you bluff, you attempt to deceive or, specifically, mislead someone. When you lie to someone who has no reason to doubt you, you aren't bluffing them, you're just lying. In order to make the false seem true, it first has to seem false. If it already seems true and you're reaffirming their incorrect belief, you aren't bluffing them.
In other words, if a minor lord approaches a major lord who knows nothing about the minor save his title and the minor lord bows appropriately and swears and oath of fealty, the major lord may not in the least be suspicious about this behaviour, especially if people are coming to his court to swear oaths and beg favours on a regular basis. Unless this is a particularly suspicious lord who would thoroughly question every oath that comes his way (diplomacy or bluff checks for everybody!) then he probably just accepts it on face value.
That doesn't mean the major lord believes the oath or that he has been mislead. The minor lord swears and oath of fealty; sure, they all do. But they don't all uphold that oath. The bluff check comes in after the betrayal happens and the minor lord tries to convince the major lord of an explanation that preserves the presumed oath. That's where the misleading happens, that's where the bluffing happens.
Lying is inherently unlawful.
The easiest way (for philosophers at least) to understand the conflict between law-chaos and good-evil is that the former represents a deontological morality and the latter represents a consequentialist morality. Being purely lawful neutral means identifying and adhering to rules, no matter what the consequences of following those rules might be. If your rule is "do not lie" then you do not lie, even if lying would bring about a "good" outcome.
Someone who does whatever it takes to advance their own goals, no matter what they have to do to advance them, is Neutral Evil. Neutral on the law-chaos axis means you neither adhere to nor intentional break laws: laws are just constructs that others have come up with and are meaningless in and of themselves, you just follow or break them as is most convenient. This might mean following the law if you can then use law enforcement to further your ends, or breaking the law if you can get away with it. Evil on the good-evil axis represents a desire to improve one's self at the expense of others, wheras Good represents a desire to improve others, sometimes (or often) at the expense of one's self.
Alignment is a role-playing guide, not a game mechanic.
Although efforts were made in earlier D&D games to wrap alignment up into the game system (eg: Paladins being restricted to lawful good), 4th edition recognizes that alignment ought not have an impact on the game. In reality, you write down an alignment on your character sheet as a guide to yourself on how to role-play your character. You encounter a situation and you think "oh, I wanted to play a lawful neutral character, how would a lawful neutral character react here?"
Meaningfully, however, being a specific alignment does not radiate that alignment out as information to others, the way being a certain race would. Each player and each NPC might have a different opinion about each character, about what alignment they think they are based on how they act. In other words, just because your player wrote "Lawful Neutral" on their sheet, they don't actually have to role-play their character as lawful neutral and you certainly don't have to have any NPC instinctively treat them as lawful neutral. If he wrote that down and isn't adhering to it, he's either a bad role-player or is evolving/changing his character as he goes; either way, this shouldn't impact the story or the game mechanics.
So what do you do in a role-play conundrum? Easy: Role-Play!
From the perspective of the game world, the character has so far sworn oaths. From the OOC perspective, the player has told you he doesn't intend to keep them. The player is doing you a huge favour: he's letting you prepare the lords' responses in advance. If a player behaved that way in game and then shared that information with me as an aside, I would interpret that as the player's desire to role-play a complex web of betrayal and intrigue, something like Game of Thrones where he tries to turn various NPCs he's sworn oaths to against one another and reap the benefits from the ensuing power vacuum.
Your first question should be: is this appropriate for my campaign? If yes, great! build it out into the campaign and engage the players with new NPCs, new betrayals, give the players opportunities to exploit NPCs and then take opportunities to betray and exploit the PCs right back. Crucially, keep the story going: don't make it look like you're being vengeful for vengeance's sake when an NPC the players trusted turns out to be plotting against them. Instead, make it into the hook for the next segment of the adventure, be it the players seeking a very personal revenge (the kind that motivates far more interesting adventure and role-play than vaguely referenced back stories) or perhaps a campaign of escape as the players find out they've bitten off more than they can chew of local politics.
If this sort of thing isn't appropriate for the campaign, prepare to stamp it out IN CHARACTER when the betraying happens. In order to enforce a lawful campaign, you have to enforce the law in the campaign world. This is actually kind of hard to do in the standard D&D 4th edition "Points of Light" campaign setting, because the world is such an inhospitable place that doesn't really lend itself to widespread policing. Without getting into the Murderous Cretins argument, ask yourself: how do lords actually enforce oaths? Hint: the answer is not "they have super high insight and bluffs on them always fail."
Perhaps the consequence for breaking an oath is a painful execution. Perhaps oathbreakers are slowly lowered into a pool of acid so every last particle is destroyed so they cannot be raised, and a team of powerful clerics or wizards are on stand-by to destroy the soul so the traitor cannot return as an undead.
Maybe you want to nip this problem in the bud by incorporating it straight into the plot. Perhaps the players witness such an execution the next time they are in town, and interact with NPCs who are muttering about how terrible it is for anyone to break an oath to the lord and how they wouldn't ever be caught associating with such an oathbreaker. Perhaps the lord calls the PC in question in to track down an oathbreaker: "by your oath sworn to me, go forth and bring this other guy who broke his oath so we can execute him in the most horrific way possible."
In-Character Actions have In-Character Consequences. No matter what you decide to do, make sure it stays in the game world. Have real and severe consequences for egregious character behaviour, and kill the character if you have to, but make sure the player knows that your goal is to keep the story going and you want their participation... just that you need them to participate in a constructive manner. Feel free to set character ground-rules if you need to: if you're playing a heroic campaign about heroes doing heroic things, then you can require that players play heroic characters; or, at least, characters that don't operate counter to the telling of a tale of heroism.
Can they? Yes. These are their own characters, after all. Nothing in the rules requires them to lay their full minds open to the DM. If it did, that would cause problems when a player decides that the PC needs to have a change of heart, for good or for ill.
Is it a good idea? Usually not. A DM who knows the PCs' true motivations can work them into the story, setting up events to help them along, or at least to make things more interesting. Players who hide their true motivations from the DM are generally shortchanging themselves, because the DM can't help them along if he doesn't know that there's something to help along.
That bit about false oaths is problematic, though. If the character was intentionally swearing oaths in bad faith, then you're right; he should indeed have been making Bluff checks. But at this stage, I'd imagine that it's too late to go back and make them retroactively: the logistics of failed checks would be too nightmarish.
My recommendation is to make the Bluff checks now, at the character's current levels (and those of the people he swore to). For each check he passes, nothing happens. On a failed check, the person he swore to starts getting a little wary: he still accepted the oath, but for some reason is starting to lose confidence in it. When and if betrayal finally comes around, these people will have some measure of preparations in place.
No, it's not a legitimate thing to do because
The Player Put the DM in a Difficult Spot
The lies the character's told--and you're certain now that they were lies1--should have been Bluff skill checks. Make those checks. Now. By yourself. Roll for the player despite his absence.
Such rolling lets you determine which folks believed the character, which folks suspected the character might not be on the up and up, and which folks knew the character was a flat-out liar. And the player won't know who fell for what. Just like real life.
Further, the player didn't say he was lying, so you needn't mention to the player that the elves' super hearing lets them hear when the heart skips during a lie, that the oath-bringers had lie-detecting magic active while the character was present, or that the king's throne puts a subtle curse on those who lie in the throne room. Obviously, don't now add such safeguards, but were such safeguards extant at the time, the character's plot would've been discovered. That's serious. And, as DM, you've to figure out why those dudes haven't acted yet on that knowledge.
A suggestion: As the character hasn't yet broken his word, there's no need actually for those who know he will to do anything so far, but the ones who know he lied will be prepared for that betrayal. Just like in real life if you know someone's planning to backstab you, you steel yourself for the inevitable betrayal not preemptively sock the dude. (That makes you the bad guy.) Same thing in a fantasy game, except assassins and thugs are much more readily available.
When you next prep for your game, go through your notes, find the lies the character's told, and roll dice to determine which of the character's lies succeeded and which failed. Then inject the failed lies into the plot when the time is right. That kind of retconning sucks, and I feel your pain.
Then Tell the Player, "Never Do That Again"
Role-playing games are shared narratives: The DM usually comes up with the plot and PCs actions usually help steer that plot. If the player wants to develop a plot, he needs to tell the DM that right now, not 6 months or whatever after he's started. A good player doesn't do this:
A good player says to the DM, "My character's planning on making an adamantine golem. Is that cool?" and the DM says, "If you can get the parts together and afford it, rock on."
A good player says at the campaign's start, "My character is a lying bastard who is only making his way through these royal courts to create alliances he'll later break because he hates these people and is extremely jealous of the wealth they possess that should be my family's." And the DM says, "Cool. That's awesome. Let's run with that."
Players sometimes forget
When playing an RPG with particularly adversarial or competitive players--who are still your friends and with whom you still like playing RPGs--sometimes the need for oneupsmanship trumps the need for communication. The desire to metaphorically win by metaphorically beating the other players or the DM is a thing. I circumvented this in a game I ran by repeatedly asking the players about characters' goals rather than their characters' steps.
It used to be like this.
Now it's like this:
Find out why players want their characters to do stuff before they start doing stuff. A DM's life is easier if you do.
I wonder, did the player change his mind or was he lying about motives from the beginning? It's legitimate for him to change is mind, but the latter is disrespectful to the group.
Has he acted on selfish beliefs yet?
If so, then you need to stop and have a conversation with the player about this inconsistency—does he prefer that his character changed his mind, or was tempted by wealth, etc. Lying to the DM doesn't make your job any easier, and he should understand that.
If not, perfect. When that happens, a lot of options become available, all of which are perfect roleplaying opportunities: possible alignment shift, bluffing through future interactions with his liege lords, unexplained wealth, dealing with shady characters, etc.
What's wrong is that if everyone sat down to play Poker but someone else wanted to play Bridge, and decided to start playing Bridge in the middle of the game, even though everyone agreed to it - that would be a case of bad behavior, right? It's really hard to tell whether someone does this kind of thing because they're deliberately griefing or if they're so used to playing with GMs who stomp their trust that they've decided the only way to meaningfully play is to lie outright and all the time, but it ultimately produces the same results of problems in play.
Avoiding stuff like this is exactly why I wrote the Same Page Tool and it might be useful to use it as a guide for your group.
On top of all of that, aren't Knights in 4E supposed to have oaths as an actual game mechanic? Meaning that, having them is part of the price you pay for the abilities the Knight gets? That would also be breaking rules on top of everything else, but obviously if you're in a position where the player doesn't trust the group enough to be honest on that level, the broken social contract outweighes the rules issues significantly.
What he tells you and what he does are two different things. His internal motivations, either as a player or as a character, are his own business and shouldn't concern you as a GM. What should concern you are his actions (player or character).
It's a nice dream to be able to plan subtleties into your campaign which are geared toward the internal motivations of the character, but that player needs to know that if he changes his mind as a player or is deceitful in any way it will not only frustrate your efforts but you as a person.
He let you believe he wants one thing, then went in another direction. You wasted your time working out ways to bring joy with his original ideas, and now have to ad-lib responses to his new direction. Sometimes players surprize a GM, and it's wonderful. Sometimes players are frustrating.
He does need to know that he has frustrated any efforts you may have made which took his original motivations into account. He as a person needs to grow.
(A "passive bluff" is a thing. It's an untrained roll.)
Yes, it's perfectly legitimate. Sortof.
First, I personally wouldn't relate lying and bluffing too closely, but if the mechanics are clear on this then yes he would have needed to make those rolls, but only if you as a GM decided it was important to do so. Lying an oath to a major NPC should definitely require effort.
Second, the player is essentially retconning his previous vision (as he told you), which is a no-no. Doing it gradually is cool. Having an about-face moment in-game which is motivated by some occurrence in-character is damned good roleplaying. Just changing his mind for no discernible reason is not.
If the rules say he should have been rolling bluffs, and he retconned that he was bluffing, then you could retconn the bluff rolls in private. See if all those people he was lying to knew all along and were themselves bluffing their acceptance of his lie. Adjust their reactions to him and plans involving him accordingly. Completely mess with that player's head and plans. Where players take no small pleasure in accidentally derailing a campaign, a GM should take pleasure in being unsurprized and able to react unflinchingly to anything the players throw at them.
That player (and every player) needs to learn that they can't go back in time and say "what I actually meant was". They're at a fixed point in the game's reality and they are not allowed to go back and change anything.
It's ok for them to have a legitimate reason to change their character's/player's mind about something, so long as there is an in-game reason for it. Ask that player what his reasons for changing his mind are. Maybe there are some subtleties that the player has only recently worked out (which is awesome!) and you can help integrate his new vision into the game.
The player can do that. The specific technical term for it is Cheating.
The DM has to keep track of the entire world, and what all the different NPCs are doing. The DM is supposed to be keeping track of sense motive checks, will saves, detect magic, zone of truth, etc. Was the dias upon which he made those oaths enchanted with a Zone of Truth spell under specific circumstances? The player doesn't know! The DM does.
What's more, this isn't just about motivations. This is about actions. A lie is not just a 'motivation'. It is an active performance. Did the liar ... fail to hold eye contact? Hold too much eye contact? What did he do with his hands? Did he sweat? Did his eyes make rapid little movements? did he fiddle with his hair? Did he twiddle his fingers? Did he rub his hands on his trouser seam? Did his intonation sound natural, or was it too fast or too firm? Did he oversell it? Undersell it?
If he did ANY of these wrong, then this is no longer a 'normal' situation, and he is not acting as expected. He is telegraphing that he's lying.
Someone skilled at seeing through lies (with a high score in their Insight) will see all of this and more, as naturally as a non-colorblind person can see that the shirt that the liar is wearing is blue. Happily, you don't have to keep track of all this because there's a convenient mechanic in place: Bluff vs. Insight.
So yes. The player can cheat. The question is: does the DM put up with cheaters at his table, and consider that to be a fair gimmick? Metagaming and all that. I've known some groups which had it considered a basic house rule that 'If you can get away with it, then you can do it,' no matter what the rules said. You have to ask yourself what you and your group feels about cheating.
Now, I have a suggestion for you.
The player's character is now under a geas.
He didn't know it, but the dias upon which he made that oath was enchanted to hold people to their words. If he breaks his oath, or even TRIES to break his oath, then he suffers the results of a broken geas until he redeems himself somehow. He gets no save because he voluntarily entered the space of the geas. And also because he didn't bother telling you that he was lying.